Thanks Carm.
Cavlino, I too like to track my car and I am considering your 275/35 suggestion w/ Michelin Pilot SS What do you run on the fronts? Any other manufacture suggestion?
Help me understand... Looking at the original OEM diameters, the Pirelli P-Zeros @ 225/40 and 265/40 are 25.3 and 26.5 inches, respectively. Say I change to the Michi SS or Bridgestone S-04. At 225/40 and 275/35 diameters for each go to 25.1 and 25.6 inches. The front has now dropped 0.2 inches and the rear by 0.9 inches. Hasn't the rake drastically changed? And thus handling is affected in an adverse way...the suspension would have to be adjusted to compensate? Is that what you gents are doing after the tire change?
Unless I'm mistaken, I think you need to halve those values as the overall diameter has changed but your drop would only be on one side of the circle (the bottom). Drops of .1 and .45 inches are insignificant in the front and fairly negligible in the rear.
Correct that you have the change in diameter to get change in ride height. Incorrect that this makes for a small change in handling. These cars are very sensitive to ride height adjustments and as little as 3mm change in rear ride height can change an understeering pig into an oversteering pig. Yet you blow off a 11mm change as if it were nothing. {Which might be ok as long as you never approach the limits of the car.}
I run the stock size in front, 225/40. I have been using yokohama advan sport tires and have been very happy with them. I did not change the suspension settings at all. The car went from slightly understeer to slightly oversteer with this rear tire change. I am used to driving an older Porsche 911 that oversteers so I prefer the oversteer tendency. I think most manufactures like to program in understeer though since that is safer for most drivers.
Mitch, I don't intentionally drive the 355 at its limits on the track due to the "what if" factor but in any case what is your recommended tires sizes front and rear for serious track duty?
Sooo, When you drive your new tires a couple hundred miles and have wore the tread down 3mm you put on new tires? Can I come pick up the "used" tires from you? From new to worn your tire diameter changes more then 1/4 inch from tread wear.
That's if you change the ride height using spring collar adjustment and you get a resultant suspension geometry change that you don't correct. The slight change of tire diameter might cause a slight weight distribution change but no more than carrying a passenger would and won't change the suspension geometry at all. If that slight weight distribution change causes any major handling difference then I'd expect the car to be completely unpredictable mid corner when transitioning from brake to gas or vice versa, and that's simply not the case. Maybe if you have a knife edge track setup it could be that sensitive, but not with the stock alignment settings I'm running.
I don't believe I used the word "handling" anywhere in my answer, nor was I flippant or "blow(ing) off" anything. I simply stated that the size values being discussed re: diameter were being calculated incorrectly. How this affects handling would need to be addressed separately. And (as Brian pointed out) you approach at least half of this (rear) value from normal use on fresh to worn tires. (And surpass it on the front)
